Methods, devices and systems, using resonating sensors for determining the values of various physical parameters in a measurement environment are well known in the art. For example, methods systems and devices for using ultrasonically activated passive sensors for sensing and measuring the values of different physical parameters within a human body or in other environments and scientific and industrial applications, have been described. U.S. Pat. No. 5,619,997 to Kaplan, incorporated herein by reference in its entirety for all purposes, discloses a passive sensor system using ultrasonic energy.
An ultrasonic activation and detection system ultrasonically activates passive sensors having vibratable parts (such as vibratable beams or vibratable membranes) which sensor(s) may be implanted in a body or disposed in other environments, by directing a beam of ultrasound at the passive sensor or sensors. The activated passive sensor(s), or vibratable parts thereof, vibrate or resonate at a frequency that is a function of the value of the physical variable to be measured. The passive sensors thus absorb ultrasonic energy from the exciting ultrasonic beam at the frequency (or frequencies) of the exciting ultrasonic beam. The amplitude of vibration of a vibratable part of such a passive sensor is maximal when the frequency of the exciting ultrasonic beam is identical to the resonance frequency of the vibratable sensor part (such as, for example a vibratable membrane or a vibratable beam included in the passive sensor). The frequency (or frequencies) at which the passive sensor absorbs and/or emits energy may be detected by a suitable detector and used to determine the value of the physical parameter.
The physical parameters measurable with such passive ultrasonic sensors may include, but are not limited to, temperature, pressure, a concentration of a chemical species in the fluid or medium in which the sensor is immersed or disposed, and the like.
If the exciting ultrasonic beam is pulsed, the ultrasonic sensor may continue to vibrate after the excitation beam is turned off. The ultrasonic radiation emitted by the activated passive sensor after turning the exciting ultrasonic beam off may be detected and used to determine the value of the physical parameter of interest.
Since more than one physical variable may influence the vibration frequency of passive sensors, a correction may be needed in order to compensate for the effects of other physical parameters unrelated to the physical parameter which needs to be determined on the measured sensor vibration frequency. For example, if pressure is the physical parameter to be determined, changes in temperature may affect the vibration frequency of the sensor. U.S. Pat. Nos. 5,989,190 and 6,083,165 to Kaplan, both patents are incorporated herein by reference in their entirety for all purposes, disclose compensated sensor pairs and methods for their use for compensating for the effects of unrelated different physical variables on the determined value of another physical variable which is being determined. For example, such compensated sensor pairs, may be used for compensating for inaccuracies in pressure measurements due to temperature changes.
U.S. Pat. No. 6,331,163 to Kaplan, incorporated herein by reference in its entirety for all purposes, discloses implantable passive sensors having a protective coating, and various types of sensor positioners or sensor anchoring devices. Such sensors may be used, inter alia, for measuring intraluminal blood pressure by intraluminal implantation of the sensor(s).
Co-pending U.S. patent application Ser. No. 10/828,218 to Girmonski et al. entitled “METHODS AND DEVICES FOR DETERMINING THE RESONANCE FREQUENCY OF PASSIVE MECHANICAL RESONATORS” filed on Apr. 21, 2004, now issued as U.S. Pat. No. 7,134,341, incorporated herein by reference in its entirety for all purposes, discloses, inter alia, methods, resonating sensors and systems, that use a Doppler shift based method for determining the resonance frequency of passive resonators. The methods, sensors and systems, may be applied, inter alia, for sensing pressure or other physical parameters in a measurement environment, such as, but not limited to the in-vivo measurement of blood pressure within a part of a cardiovascular system.
While all the above examples are related to passive resonating ultrasonic sensors, many other types of resonating sensors including both active and passive sensors are known in the art for measurement of various different physical parameters. Such sensors have in common the use of one or more resonating vibratable structures or parts, such as, for example vibratable membranes or beams or the like, which may be passively or actively vibrated. The resonance frequency of the resonating structure of such sensors changes as a function of the physical variable to be determined and may be sensed or measured in various different ways and used to determine the value of the physical variable. Examples of such sensors are the active ultrasonic sensor disclosed in U.S. Pat. No. 6,461,301 to Smith. Additional sensor types are disclosed in U.S. Pat. No. 6,312,380 to Hoek et al.
A common problem when resonating sensors such as, but not limited to, the sensors described above are implanted within a living body is the deposition of tissue or other materials of biological origin on the sensor or on parts thereof. For example, various substances or living cells may attach to the surface of the resonating sensor or to various parts thereof and adjacent tissues may cause the deposition of a layer or film of material and/or cells, and/or tissues on the sensor's surface. The deposition of tissues or other biological materials on the vibratable part of the sensor, such as (but not limited to) the vibratable membrane of a passive (or active) resonating sensor may cause changes in the vibratable membrane (or the other vibratable part) resonance characteristics such as, inter alia, the resonance frequency, sensitivity to stress, and vibration amplitude of the vibratable membrane. Such changes may adversely affect the sensor's performance and the accuracy of the determination of the physical variable which is to be determined.
Similarly, when a resonating sensor is disposed within a fluid or gas or other medium or measurement environment which contains various substances (such as, for example, within a chemical reaction mixture in a reactor or in a measurement environment containing sprays or aerosols or the like), deposition of liquid or solid material or particles on the vibratable part of the resonating sensor may similarly affect the resonance characteristics of the vibratable part of the sensor with similar adverse effects on the sensor's performance.